That’s “Festival” and the Red Hour. Fortunately, the will of Landrieu will be overthrown on Saturday.
“The U.S.S. Enterprise is investigating Beta III, where the U.S.S. Archon disappeared over 100 years before. When the landing party exhibits strange behavior, Kirk sends another party down to investigate. They find the culture on Beta III is quiescent, with no creative tendencies. The entire culture is controlled by a group of ‘lawgivers’ known as “The Body” which is, in turn, controlled by the omniscient Landru. The inhabitants change from normal, peaceful people to a violent mob at the coming of the Red Hour. This ‘Festival’ is the society’s only outlet for the tyrannical hold that Landru has over them at all other times.”
Star Trek: Episodes: The Return of the Archons:
http://www.startrek.com/startrek/view/series/TOS/episode/68704.html